Reactive power control can be used for optimizing reactive power flow within an electric power system. A Static VAR (Volt-Ampere Reactive) Compensator (SVC) is an arrangement frequently used within such power systems for handling disturbances within the power system by means of reactive power. The SVC counteracts voltage drops in the power system by providing reactive power and is often able to handle overvoltages by absorbing reactive power. In short, the SVC is used for maintaining the voltage of the power system at a desired level by adjusting the reactive power flow.
To this end, the SVC typically comprises thyristor-switched capacitors (TSC) and thyristor-controlled inductive elements, also denoted reactors (TCR). These components of the SVC are controlled so as to provide the desired reactive power. In particular, if the power system's reactive demand is capacitive (leading), the SVC uses the reactors to consume VARs from the power system to which it is connected, thereby lowering the voltage of the power system. If the reactive power demand of the power system is inductive (lagging) the capacitors are used for supplying VARs to the power system, thereby increasing the power system voltage. The SVC further comprises a control system for controlling these functions.
FIG. 1 illustrates schematically a known SVC 1. The SVC 1 is connected to a power system 2, in the following denoted an AC grid 2, via a power transformer 3. The SVC 1 comprises a bank of thyristor-switched capacitors (TSC) 4, thyristor-controlled reactors (TCR) 5 and harmonic filters 6. The SVC 1 further comprises a control system 7 for regulating the reactive power input from the SVC 1 to the AC grid 2.
At dramatic voltage drops in the AC grid 2, the function of the SVC 1 deteriorates in that less reactive power can be output therefrom. This deteriorated performance of the SVC 1 stems from the impedance characteristics of the shunt connected TSC 4 and TCR 5 of the SVC 1. If neglecting voltage drops over the power transformer 3, this can be approximated by the following equation:Q=B*(τ*U)2 where Q is the generated reactive power of the TSC on a secondary side of the power transformer 3, B is the admittance of the shunt capacitor of the TSC, U is the SVC controlled AC voltage in the AC grid bus on the primary side of the power transformer 3 and τ is the power transformer ratio.
τ is equal to 1.0 for the power transformer 3 of the SVC 1, and it can thus be seen that the reactive power output Q is related to the square of the AC voltage. The deteriorated performance of the SVC 1 at large voltage drops, mentioned earlier, can thus easily be realized. As the SVC reactive power output Q approaches its rated values, the SVC looses active control.
Today, to the extent that the above described shortcoming is at all addressed, the SVC would most likely be overrated. In particular, additional TSC steps could be introduced. However, overrating power system components is in general a most inefficient solution as such components often are expensive.
From the above, it is clear that there is a need for an improvement on this situation in this field of technology.